One of Seatle's newest and most-prominent office towers has changed hands for the second time since its construction in 2006.

Northwestern Mutual announced Friday that it has sold the 42-story, 872,000-square-foot Russell Investments Center, at 1301 Second Ave., to CommonWealth Partners, a privately held real estate investment, development and management firm based in Los Angeles, for $480 million. Northwestern said the sale "is believed to be the largest single asset office sale in the Western United States since 2006."

The tower started life as the WaMu Center. Northwestern Mutual bought it in the fall of 2009 for just $115 million, after Washington Mutual's collapse and takeover by JPMorgan Chase. Northwestern said the building was "virtually empty" at the time but is now 95 percent leased.

The tower was renamed as part of Russell Investments' move of its headquarters from Tacoma to Seattle. Boeing, Nordstrom, Dendreon, JP Morgan Chase and Zillow are also among the companies with long-term leases, which should not be affected by the sale.

The real estate firm CBRE, which handled the sale on behalf of Northwestern Mutual, said in a separate news release that the building was just 30 percent leased 24 months ago.

CBRE Vice Chairman Kevin Shannon said:

This is a best-in-class building that is widely considered one of the top three assets in the entire Seattle market. Its premiere location, unobstructed water views, LEED Platinum status, and exceptional credit tenant base made for a highly competitive bid process. There were 34 prospective buyer tours of the asset, which is highly unusual for a deal of this size and demonstrates the tremendous amount of capital that exists for core CBD assets. This is especially true in rising markets, like Seattle, that are experiencing strong job growth resulting in aggressive rent growth.

CommonWealth Partner Rick Lewis, Partner said the tower and nearby Safeco Center, which the firm already owned, "are well-positioned to benefit from the continued momentum in the leasing market." He said the firm has more than $5 billion to invest and will continue to look for opportunities in the Seattle area and other major U.S. markets over the next several years.